1. Technical Field
This invention relates generally to the field of textile coating machines and more particularly to an apparatus and method for applying a foamed coating to a traveling textile substrate.
2. Background Information
The processing of textile fabrics and similar substrates typically involves application of various coating materials to the fabric to achieve specific purposes. For example, binder coatings are used on some textile substrates to improve the structural integrity of the substrate and dye coatings are often used on textile substrates to achieve a desired fabric color. Regardless of the particular coating being applied, two important and often competing considerations must be addressed. First, it is important that the required amount of coating material be uniformly applied to the textile substrate. Failure to uniformly apply sufficient coating material to the substrate could result in such deficiencies as insufficient structural integrity of the textile substrate in the case of binder coating processes or inconsistent or variable coloration in the case of a dye coating process. Second, coating material must be efficiently applied. Using more coating material than required is wasteful and therefore costly and applying coating materials in an inefficient manner, such as spraying, can result in environmental pollution and necessitate costly measures to reduce the environmental impact of the coating process.
Applying a uniform coating to a textile substrate in an efficient manner is particularly difficult when the coating material is a material such as latex or any other material that is film-forming at atmospheric pressure. These coating materials typically have higher viscosities than many textile coating materials and can also dry inside coating machinery and thereby clog or reduce the flow in that machinery. When coating with film-forming coating materials, therefore, precautions must be taken when the substrate line stops or when a coating process is completed. The coating apparatus must be sufficiently cleansed of the film-forming material after operation or the machinery must be left in such a condition that the coating material is not allowed to dry on the inside walls of the applicating machinery. This is particularly important in the area of the applicator nozzle, which is sized to ensure that a specific amount of material is applied. Any film buildup on the walls of the applicator nozzle can either clog the nozzle or result in delivery of less than the designed amount of coating material.
There are several known methods of applying coatings to a textile substrate. One such method is immersing a moving substrate in a bath of coating material. This method usually applies more coating material than required to the traveling substrate and thus it is often necessary for the substrate to undergo subsequent processes, such as nip rolls or dryers, to remove excess coating material and moisture. This immersion method, therefore, is inefficient because too much coating material is applied to the substrate and wasteful because some coating material is lost in the subsequent process of removing the excess material.
Another known method of coating a textile substrate is to apply coating material to the surface of a traveling substrate and allow the coating material to impregnate the substrate by absorption or by capillary action. But absorption and capillary action can result in nonuniform application of coating material, especially when using viscous coating materials such as latex because the effectiveness of these methods depends in large part upon the structure or composition of the substrate. A non-uniform substrate often results in non-uniform absorption or capillary coating. Moreover, relying upon absorption or capillary action also results in more coating material being applied to the surface of the substrate than required to ensure that enough coating material is available for penetration into the fabric. The excess coating materials must then be removed from the fabric using devices such as a doctor blade or knife edge.
In recognition of the limitations of capillary action coating, various additional coating techniques have been developed. For example, one variation involves the application of vacuum to the substrate in order to draw coating material deposited on one surface into the substrate. Another variation involves directing the coated substrate through a series of nip rolls to force coating material into the substrate. While these variations are perhaps more efficient than solely coating a textile fabric, they can also produce such undesirable results as the lack of uniform distribution of coating material and waste of coating material.
A number of attempts have been made to overcome the drawbacks of the above-mentioned coating processes and many of these attempts involve the use of foamed coating materials. Foamed coating methods are advantageous because they allow the delivery of coating material to a substrate using less water than non-foamed coating procedures. This results in less runoff waste liquids—which require proper disposal precautions—and less energy use because subsequent machinery to remove excess water from fabrics is eliminated using foam coating techniques.
But even foamed coating material have disadvantages. For example, it is often difficult to achieve uniform application of foamed coating material to a substrate because the results of conventional foamed coating methods often vary depending on the structure of the textile substrate or the viscosity of the coating material.
Another problem with conventional foamed coating methods is how to accommodate disruptions or stoppages in the textile processing line. This difficulty results from the fact that foamed material breaks down over time and becomes nonuniform if pressure is ever allowed to equalize in the distribution path. When processing of a textile substrate is halted, as would be required to accommodate machine stoppages upstream or downstream of a traveling textile substrate, to correct substrate breakage, or to change substrate materials, then either the foam applicator must be shut—thereby risking equalizing pressure in the foam distribution system—or foam flow can be continued—thereby wasting coating materials and wasting that portion of the traveling substrate upon which the excess coating material accumulates during the line stoppage.
Complicating the problem even further is the fact that many textile mills process fabric face-down. This procedure allows workers clear visibility of the processes occurring to the back side of the fabric but face-down processing of textile fabrics is problematic for coating machines dispensing film-forming coating material because when the fabric line stops or is shut down there is the risk that the film-forming coating will dry in the applicator nozzle or on the inner surface of the coating delivery piping. If the coating material is a foamed film-forming material, the problem is worse still because there is the added difficulty of not allowing the foamed material to equalize pressure throughout the distribution line. Furthermore, when operations are completed, it is essential that the film-forming coating material be properly cleansed from the applicator components, which are necessarily facing downward in order to apply the coating to the reverse side of a face-down fabric as it travels along the processing line.
It would therefore be desirable for a coating apparatus to have the capability to uniformly dispense a foamed film-forming coating material along the width of a traveling face-down substrate while at the same time having the ability to accommodate temporary line stoppages as well as long-term production line halts without resulting in nozzle clogging or coating material buildup on the inside of the coater walls. This capability would desirably be independent of the structure of the substrate and independent of the coating material used. It would also be desirable for such a machine to be easily cleansable without necessitating time-consuming disassembly and/or manual part cleaning.
There are numerous designs of foam applicators existing in the art, several of which are capable of delivering a foamed coating of film-forming material. But these applicators have not achieved all of the desirable characteristics of a coating apparatus discussed above. For example, U.S. Pat. No. 4,562,097 to Walter et al. discloses a method of treating a porous substrate by applying a foamed treating composition on the surface of the substrate with an applicator nozzle in contact with the moving substrate. While latex is disclosed as a suitable treating composition, the Walter et al. patent does not appear to specifically address the inherent film-forming problem associated with latex application or a method of cleansing such a film-forming material from the applicator when not in use.
U.S. Pat. No. 4,023,526 to Ashmus et al. discloses foam applicator heads for the application of a chemical treatment. Uniformity of foam application in this device, however is effected by the angle and contact between the substrate and the inward taper of the downstream nozzle lip. Also, as in the previously discussed patent, the Ashmus patent does not specifically address the problem of film formation during line stoppages or the problems incurred when using the disclosed applicator head in a fabric line to treat fabric face-down.
U.S. Pat. No. 5,219,620 to Potter et al. discloses a foam applicator intended for use in a fabric line that processes fabric face-down. The Potter et al. foam applicator is an arcuate assembly that is pressed tightly against the traveling fabric by pneumatic or hydraulic cylinders over a wrap angle in order to assure uniform pressure and seal of the applicator against the fabric. Such an apparatus would therefore be undesirable for use in applying a film-forming material to a traveling textile substrate that could not withstand applicator pressure without breaking the substrate. Moreover, this patent does not appear to include latex or other film-forming compositions among the intended treating compositions and thus it too does not address the unique problem associated with such compounds.
While each of the patents discussed above describe an apparatus having certain desirable features, it is clear that a better foam coater is needed in the art. More particularly, there is a need for a foam coater apparatus capable of uniformly applying a metered amount of foamed, film-forming coating material to a traveling substrate in a face-down production line regardless of the structure of substrate and regardless of the viscosity of the coating material. The need is also for such a coater to have the ability to accommodate temporary line stoppages without wasting a significant amount of coating material when the line production recommences and to accommodate long-term line stoppages without allowing film formation to clog the applicator nozzle or associated foam delivery system piping. Finally, such a coater should have the ability to be cleansed of foamed material in an efficient and simple manner. Indeed, a coater possessing all of these attributes would be able to efficiently deliver a specified amount of film-forming coating material to a traveling substrate without wasting significant amounts of coating material and, when no longer needed, such a machine would be able to stop operations without the risk of film formation clogging the applicator nozzle.